I have a passwd file with 3 users belonging to the the root group (gid=0), but the group file does not list these users as members of the root group?
Shoud I be worried and apart from manually changing it, how can it be remediated?
No need to worry. The primary group of a user is defined by the entry in passwd. Any user noted in /etc/group has this group as a secondary group. You can check what groups a user belongs to by using the id command.